EC2301 DIGITAL COMMUNICATION V Sem ECE – R.Vanitha Asst.Prof./ECE Page 1 MAHALAKSHMI ENGINEERING COLLEGE TIRUCHIRAPALLI – 621213 QUESTION BANK DEPARTMENT: ECE SEMESTER: V SUBJECT CODE / Name: EC2301 – DIGITAL COMMUNICATION UNIT-V BASEBAND SIGNAL TANSMISSION AND RECEPTION PART -A (2 Marks) 1. Define QAM and draw its constellation diagram. ? [AUC NOV/DEC 2010] 2. A binary frequency shift keying system employs two signaling frequencies 1f and 2f. The lower frequency 1f is 1200 Hz and signaling rate is 500 Baud. Calculate 2f. ? [AUC NOV/DEC 2010] For binary FSK baud=fb Fb=500Hz Consider the FN modulation index(h) of 1 in FSK Fm-fs/Fb =h=1 Fm-fs=fb Fs=f1=1200HZ
Transcript
EC2301 DIGITAL COMMUNICATION V Sem ECE – R.Vanitha Asst.Prof./ECE Page 1
MAHALAKSHMI
ENGINEERING COLLEGE
TIRUCHIRAPALLI – 621213
QUESTION BANK
DEPARTMENT: ECE SEMESTER: V
SUBJECT CODE / Name: EC2301 – DIGITAL COMMUNICATION
UNIT-V BASEBAND SIGNAL TANSMISSION AND RECEPTION
PART -A (2 Marks)
1. Define QAM and draw its constellation diagram. ? [AUC NOV/DEC 2010]
2. A binary frequency shift keying system employs two signaling frequencies 1f and 2f.
The lower frequency 1f is 1200 Hz and signaling rate is 500 Baud. Calculate 2f. ?
[AUC NOV/DEC 2010]
For binary FSK baud=fb
Fb=500Hz
Consider the FN modulation index(h) of 1 in FSK
Fm-fs/Fb =h=1
Fm-fs=fb
Fs=f1=1200HZ
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Fm-1200Hz=500Hz
Fm=1700Hz,f2=fm=1700Hz
3. Draw the PSK waveform for 011011. . [AUC APR/MAY 2011]
4. What is meant by coherent detection system? . [AUC APR/MAY 2011]
In coherent ASK, correlation receiver is used to detect the signal. Locally generated
carrier is correlated with incoming Ask signal. The locally generated carrier is in exact
phase either transmitted carrier. Coherent Ask is also called synchronous ASK
5. Why is PSK always preferable over ASK in coherent detection? [AUC NOV/DEC
2011]
ASK is on-off signaling where as the modulated carrier is continuously transmitted in
PSK. Hence peak power requirement is more in ASK, where it is reduces in PSK
6. Differentiate between coherent and non-coherent detection [AUC NOV/DEC 2011]
[AUC APR/MAY 2012]
In coherent detection the local carrier generated at the receiver is phase locked with
the carrier at the transmitter. Hence it is also called synchronous detection. In non
coherent detection the local carrier generated at the receiver not be phase locked
with the carrier at the transmitter. It is simple, but it has higher probability of error.
7. What are the drawbacks of binary PSK system? [AUC APR/MAY 2012]
It is difficult to detect +b(t) and –b(t) because of squaring in the receiver
Problem, of ISI and inter channel interference are present
8. A BPSK system makes errors at the average rate of 1000 errors per delay. Data rate
is 1 kbps . The single-sided noise power spectral density is 10-20 W/Hz. Assuming
the system to be wide sense stationary, what is the average bit error probability?
[AUC NOV/DEC 2012]
24*60*60=86400sec
86.4*106
Bir error probability Pe=100/86.4*106
=1.1157*10-6
9. What is meant by memory less modulations? [AUC NOV/DEC 2012]
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When the digital symbol modulates amplitude, phase or freauwny of the carrier
without any refeencee to perious sysmol, it is called memory less modulations.
Eg.:ASK,PAS,FSK,PSK
10. What is Signal constellation diagram?
Suppose that in each time slot of duration T seconds, one s2(t), . . sM(t) is
transmitted with equal probability, 1/M For geometric representation, the signal si (t), i
= 1, 2, ... , M, is applied to a bank of correlators. The correlator outputs define the
signal vector si. The set of message points corresponding to the set of transmitted
signals {si(t))} i=1..M is called a signal constellation.
11. What is meant by DPSK?
In DPSK, the input sequence is modified. Let input sequence be d(t) and output sequence
be b(t). Sequence b(t) changes level at the beginning of each interval in which d(t)=1 and it
does not changes level when d(t)=0.
When b(t) changes level, phase of the carrier is changed. And as stated above, b(t) changes
t=its level only when d(t) =1. This means phase of the carrier is changed only if d(t)=1.
Hence the technique is called Differential PSK.
12. Explain coherent detection?
In coherent detection, the local carrier generated at the receiver is phase locked with the
carrier at the transmitter. The detection is done by correlating received noisy signal and
locally generated carrier. The coherent detection is a synchronous detection.
13. What is the difference between PSK and FSK?
In PSK, phase of the carrier is switched according to input bit sequence. In FSK frequency of
the carrier is switched according to input bit sequence. FSK needs double of the bandwidth
of PSK.
14. What is meant by coherent ASK?
In coherent ASK, correlation receiver is used to detect the signal. Locally generated carrier is
correlated with incoming ASK signal. The locally generated carrier is in exact phase with the
transmitted carrier. Coherent ASK is also called as synchronous ASK.
15. What is the major advantage of coherent PSK over coherent ASK?
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ASK is on-off signalling, where as the modulated carrier is continuously transmitted in
PSK. Hence peak power requirement is more ASK, whereas it is reduced in case of
PSK.
16. Bring out the difference between coherent & non coherent binary modulation
scheme.
a. Coherent detection:
In this method the local carrier generated at the receiver is phase locked with the carrier at
the transmitter. Hence it is called synchronous detection
b. Non coherent detection:
In this method, the receiver carrier need not be phase locked with transmitter carrier.
Hence it is called envelope detection
17. Write the expression for bit error rate for coherent binary FSK.
Bit error rate for coherent binary FSK is given as,
Pe = 1/2erfc√0.6E/No
18. Highlight the major difference between a QPSK & MSK signal.
MSK signal have continuous phase in all the cases, where as QPSK has phase shift of π/2
or π.
19. What is the error probability of MSK & DPSK?
Error probability of MSK: Pe = 1/2erfc√E/No
Error probability of DPSK: Pe = 1/2e-Eb/No
20. In minimum shift keying what is the relation between the signal frequencies &
bit rate.
Let the bit rate be fb and the frequency of carrier be f0. The higher and lower MSK signal
frequencies are given as,
fH = f0 + fb/4
fL = f0 - fb/4
PART -B (16 Marks)
1. (i) distinguish coherent and non-coherent detection. (4) (ii) Explain non
coherent detection methods of binary frequency shift keying scheme. (12) ?
[AUC NOV/DEC 2010]
In coherent detection the local carrier generated at the receiver is phase locked with
the carrier at the transmitter. Hence it is also called synchronous detection. In non
coherent detection the local carrier generated at the receiver not be phase locked
with the carrier at the transmitter. It is simple, but it has higher probability of error.
Frequency Shift Keying (FSK) modulation is a popular form of digital modulation used
in low-cost applications for transmitting data at moderate or low rate over wired as
EC2301 DIGITAL COMMUNICATION V Sem ECE – R.Vanitha Asst.Prof./ECE Page 5
well as wireless channels. In general, an M-ary FSK modulation scheme is a power
efficient modulation scheme and several forms of M-ary FSK modulation are
becoming popular for spread spectrum communications and other wireless
applications. In this lesson, our discussion will be limited to binary frequency shift
keying (BFSK).
Two carrier frequencies are used for binary frequency shift keying modulation. One
frequency is called the ‘mark’ frequency (f2) and the other as the space frequency (
f1). By convention, the ‘mark’ frequency indicates the higher of the two carriers used.
If Tb indicates the duration of one information bit, the two time-limited signals can be
expressed as :
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2. Explain binary PSK and QPSK with corresponding equations and constellation
diagrams. (8) (ii) Obtain the probability of bit error rate for coherently detected BPSK
and compare its probability of bit error rate performance with QPSK scheme. (8)?
[AUC NOV/DEC 2010]
BPSK is a simple but significant carrier modulation scheme. The two time-limited
energy signals s1(t) and s2(t) are defined based on a single basis function ϕ1(t) as:
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GENERATION AND COHERENT DETECTION OF BPSK SIGNALS
(i) Generation
To generate the BPSK signal, we build on the fact that the BPSK signal is a special case of
DSB-SC modulation. Specifically, we use a product modulator consisting of two components
(i) Non-return-to-zero level encoder, whereby the input binary data sequence is encoded in
polar form with symbols 1 and 0 represented by the constant-amplitude (ii) Product
modulator, which multiplies the level encoded binary wave by the sinusoidal carrier of
amplitude to produce the BPSK signal. The timing pulses used to generate the level-
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encoded binary wave and the sinusoidal carrier wave are usually, but not necessarily,
extracted from a common master clock.
(ii) Detection
To detect the original binary sequence of 1s and 0s, the BPSK signal at the channel output
is applied to a receiver that consists of four sections
(i) Product modulator, which is also supplied with a locally generated reference signal that is
a replica of the carrier wave
(ii) Low-pass filter, designed to remove the double-frequency components of the
product modulator output (i.e., the components centered on ) and pass the zero-frequency
components.
(iii) Sampler, which uniformly samples the output of the low-pass filter at where ; the local
clock governing the operation of the sampler is synchronized with the clock responsible for
bit-timing in the transmitter.
(iv) Decision-making device, which compares the sampled value of the low-pass filter’s
output to an externally supplied threshold, every seconds. If the threshold is exceeded, the
device decides in favor of symbol 1; otherwise, it decides in favor of symbol 0. levels: and _
respectively
.
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3. Derive bit error probability due to coherent ASK, PSK and FSK system. Compare the
performance of these systems . [AUC APR/MAY 2011] Compare the performance of
various coherent and non-coherent digital detection systems. [AUC NOV/DEC 2012]
Binary ASK
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4. (i) Discuss QPSK signaling. (ii) Derive the bit error probability due to QPSK receiver.
Compare the performance of QPSK receiver with that of PSK receiver. . [AUC
APR/MAY 2011]
In a sense, QPSK is an expanded version from binary PSK where in a symbol
consists of two bits and two orthonormal basis functions are used. A group of two bits
is often called a „dibit‟ . So, four dibits are possible. Each symbol carries same
energy. Let, E: Energy per Symbol and T: SymbolDuration = 2.* Tb, where Tb:
duration of 1 bit.
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In QPSK system the information carried by the transmitted signal is contained in the
phase.
QPSK Receiver:- The QPSK receiver consists of a pair of correlators with a
common input and supplied with a locally generated pair of coherent reference
signals ᶲ1(t) & ᶲ2(t)as shown in fig(b).The correlator outputs x1 and x2 produced in
response to the received signal x(t) are each compared with a threshold value of
zero. The in-phase channel output : If x1 > 0 a decision is made in favour of
symbol 1 x1 < 0 a decision is made in favour of symbol 0 Similarly quadrature
channel output: If x2 >0 a decision is made in favour of symbol 1 and x2 <0 a
decision is made in favour of symbol 0 Finally these two binary sequences at the in
phase and quadrature channel outputs are combined in a multiplexer (Parallel to
Serial) to reproduce the original binary sequence
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Probability of error:- A QPSK system is in fact equivalent to two coherent binary
PSK systems working in parallel and using carriers that are in-phase and quadrature.
The in-phase channel output x1 and the Q-channel output x2 may be viewed as the
individual outputs of the two coherent binary PSK systems. Thus the two binary PSK
systems may be characterized as follows.
.
5. Explain the concept of coherent BPSK with transmitter and receiver block diagrams
and obtain the expression for probability of error. [AUC NOV/DEC 2011]
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6. A set of binary data is sent at the rate of Rb=100 kbps over a channel with 60dB
transmission loss and power spectral density η=10-2 w/Hz at the receiver. Determine
the
transmitted power of error probability Pe=10-3 for the following modulation schemes
